1. Field
The present embodiments generally relate to the treatment of process water streams. More particularly, embodiments of the present invention relate to the treatment of process water streams generated by a hydrocarbon gasification process.
2. Description of the Related Art
Gasification is a high-temperature process for converting carbonaceous materials into carbon monoxide and hydrogen gas. Since the mixture of carbon monoxide and hydrogen is often used for the subsequent synthesis of both organic and inorganic chemicals, the gas is often referred to as “synthesis gas” or, more succinctly “syngas.” Syngas can be used as a fuel to generate electricity or steam, as a raw material for chemical production, and as a source of hydrogen.
Typical feeds to gasification include petroleum-based materials that are neat or residues of processing materials, such as heavy crude oil, bitumen recovered from tar sands, kerogen from oil shale, coke, and other high-sulfur and/or high metal-containing residues; gases; and various carbonaceous waste materials. Dry or slurried feedstock is reacted in the gasifier in a reducing atmosphere and at high temperatures and pressures. The resulting syngas typically contains about 85 percent of the feed carbon content as carbon monoxide and hydrogen, with the balance being a mixture of carbon dioxide, water vapor and methane. The syngas can exit the gasifier at temperatures from about 1,000° F. to about 2,900° F. Contaminants present in the gasifier feedstock, such as sulfur containing compounds, halogenated compounds, and ammonia containing compounds will be present in the high-temperature syngas exiting the gasifier.
This high-temperature syngas can be purified for subsequent use as a process feedstock, however most process equipment used to clean impurities and contaminants from the syngas operate at temperatures far lower than the gasifier exit temperature. Thus, the syngas requires substantial cooling prior to subsequent processing. First stage cooling is typically accomplished using non-contact heat exchange with water, cooled syngas, or other relatively cool media. Reducing the temperature of the syngas below the dew point condenses a portion of the water present in the syngas. As the water vapor condenses, contaminants present in the syngas, such as hydrogen sulfide, hydrogen chloride, ammonia, and hydrocarbons, are absorbed into the condensate. Direct cooling methods such as wet venturi scrubbing, quenching, or the equivalent, are often used to effectuate second stage cooling of the syngas. While direct contact cooling can efficiently drop the syngas temperature to about 400° F. to about 700° F., the voluminous quantity of cooling water contacting the syngas entrains significant additional contaminants. Thus, both the condensate from the non-contact first cooling stage and the contact cooling water from the second cooling stage require treatment prior to discharge.
Frequently, the process condensate and contact cooling water are combined and treated in large-scale industrial or publicly-owned treatment facilities. However, where gasification processes are employed in remote locations such dedicated treatment infrastructure is often unavailable or lacks sufficient hydraulic capacity to adequately handle the large, continuous, volumes of water generated.
There is a need therefore for a method for efficiently and economically treating the relatively large volume of water generated by a hydrocarbon gasification process.